It's common knowledge that wildlife breeds in the spring. When it comes to medium and large mammals, however, common knowledge is often wrong. Mating season peaks in mid-winter for many mammals, and some species actually mate a year in advance of giving birth.
The fisher, for example, is a large member of the weasel family. They mate in early spring, but shortly after fertilization, the tiny embryos remain in the uterus but do not implant on the uterine wall. After 10 to 11 months, these "blastocysts" finally implant and a functional gestation period of 30 to 60 days follows. Two or three pups are born in March or April, almost a full year after mating. "Delayed fertilization" is characteristic of several other members of the weasel family and black bears.
River otters mate in March or April and give birth to two or three pups the following February or March. Embryos do not implant on the uterine wall for up to 10 months, followed by an actual pregnancy of about 50 days.
Black bears mate in June or early July, but embryos do not implant until the female enters hibernation. Two or three cubs are born in January while still in the winter den.
Delayed implantation permits females to meet the energetic demands of pregnancy while food is abundant. And young animals leave their dens just as their foods become relatively easy to find.
The peak of the striped skunk breeding season is underway right now. From late February through early March, skunks roam the countryside searching for mates. This may explain why we see so many dead skunks along country roads this time of year. After a gestation period of about 63 days, four to six kits are born in April or May.
Though small rodents such as mice and voles breed frequently, larger rodents devote more time and energy to each reproductive effort. Gray and fox squirrels, for example, mate in January and after a 44-day gestation period give birth to four or five young. The young do not leave the nest until they are 10 to 12 weeks old. A second litter is often born in July or August. The first litters are usually born in tree cavities, while summer litters are often raised in leaf nests.